Yes. Locate the user via the admin panel.
Look in the bottom left corner of the frame that holds the info, there should be a field called "Actions:" and below that, there are a bunch of links. The most left link should be called "Edit Profile".

If you have an older install of FUDforum, it will be a huge text saying >> Change User's Profile <<.

When in the users register page, change the email adress and click the Update/Save button - The forum will tell you to insert your ADMIN password in the password field, do that, hit update again, and boom. Done.